i don't think you can crossfire those together... at least if you did it would clock the 6670 to the 6550 speeds... you need identical cards to crossfire.
As long as the cards share the same GPU, they can be used in Crossfire. For example, you can Crossfire a 6850 with a 6870 since they both use the Barts GPU. Likewise, since the 6670 and the 6550D both use the Turks GPU, they can be Crossfired.
As far as the poor scaling, dudewitbow is right, some games just won't do very well with it.